Heat recoverable sleeving is often used to protect joins in electrically conducting cables and in optical fibre signalling lines. In use, a sleeve is passed over one end of a cable, for example a cable having multiple pairs of signalling wires and, once jointing of one cable to another cable has been completed, the sleeve is relocated over the joint area, the sleeve spanning the ends of the respective insulating jackets of the cables. Alternatively after the joint has been formed a split sleeve is positioned over the jointing area and is sealed using a hot melt adhesive. Heating the sleeve causes it to shrink to grip tightly around the cable jacket and the joint area thereby effecting water resistance and insulation. Care must be taken during the shrinking operation to ensure that air entrapment is limited.
Thus, for example, if the ends of a sleeve are recovered too rapidly so that sealing of the sleeve to the insulating cable jackets prevents air from escaping then the central portion of the sleeve may be prevented from shrinking fully onto the joint area by the entrapped air. Accordingly skilled operators are employed to effect the shrinking operation usually by applying a gas torch to the sleeve area, moving the torch from the center toward the ends.
However, as will be appreciated, many such jointing operations are performed in restricted areas such as in underground cable chambers where the effects of combustion products may be dangerous to the operator. Furthermore, the use of a naked flame in such chambers, in which a build up of combustible or explosive gases is possible, should preferably be avoided.
One solution to the above problem has been to incorporate sacrificial heating elements into the sleeving (see for example EP38659 and EP52476) or to lay a sacrificial heating element into the joint prior to positioning the sleeve for example using the method of WO 84/03346. Application of an electrical current to the heating elements then effects shrinkage.
However, sleeving including such elements is necessarily more expensive to produce since the heating wires are not available for further use and control of the application of heat to ensure central shrinkage prior to end shrinkage may be difficult to achieve.
One method of ensuring distribution of heat in a suitable manner for use in a fixed situation, for example at a manufacturing site, is disclosed in EP179606. In the aforementioned specification there is disclosed a bench mounted heating device having a centrally disposed heating element and an elongate plate of thermally conductive material. Thus, by placing a sleeved joint on the plate and then applying power to the heating element the central portion reaches recovery temperature very quickly while the outer portions reach that temperature more gradually. Such a device may not be suitable for use on sleeves which have a large diameter since only one area, that which is in contact with the plate, would readily reach the required temperature. Furthermore such a device is not necessarily suited to be adapted for use in the field.